Rohde & Schwarz HM8118 Quick Start Manual

HM8118: Measurement of capacitors and
coils
From the expert's desk
Products:
HM8118
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1. Introduction:
The HM8118 is an LCR Bridge with a basic accuracy of 0,05%. The
test frequency can be set from 20Hz up to 200kHz.
This Application Sheet helps to perform first measurements on capaci-
tors and coils without having a doubt concerning the results.
It does not claim to be complete. All information has been compiled
with care. However errors can't be ruled out.
2. General information about the measurement principle
The HM8118 is not a typical Wien, Maxwell or Thomson bridge based
instrument. It measures the impedance Z and the phase angle Φ with
an AC stimulus signal. In combination with other known parameters
(e.g. frequency) the desired characteristics can be calculated.
When performing measurements always remember, that e.g. the ca-
pacity varies with certain parameters, specially over the frequency. Al-
so aging, temperature, the level of the measurement signal and addi-
tional bias are dependency factors.
The equivalent circuit of each DUT always contains inductive, real and
capacitive parts. E.g. a capacitor will have inductive and real parasit-
ics.
This means, that ideal Φ (90°=pure inductive, 0°= pure ohmic, -90°
=pure capacitive) will never be reached.
It is also important, that due to the opositional parasitic parts each
measurement contains a certain inaccuracy being a systematic meas-
urement error.
Extraction from the user manual: Phase angle and integral parts of L, C, R
As an example a bigger coil always contains a parasitic ohmic
and capacitive part in addition to the inductive essential part.
The inductive part will prevail, while the capacitance can not
directly be measured with the HM8118. However this coil will
have a self resonance showing the capacitive part to be pre-
sent.
3. Test assembly
It is important to allways keep in mind, that the test assembly
may have appreciable influence to your measurement. When
talking about capacitors in the range of picofarads, the position
of the clamps has definitely influence to the measurement.
You can easily give it a try by performing the following steps:
- use the basic setting (1kHz / Single Mode)
- shorten the clamps and perform a short calibration
- lay down the clamps with at least 20cm difference and per-
form an open calibration
- change to C-D Mode
- Now hold the clamps, that the tips are just not touching and
check the measurement result.

I can achieve more than 3pF with this method
So allways choose the best test adaptor for your application.
The HZ181 is a great tool for leaded (through hole) compo-
nents avoiding such distance issues:
4. Alignment of the test structure
We recommend to align the instrument on all available fre-
quencies (20Hz – 200kHz in 69 Steps) when measuring un-
know devices. After choosing the right test assembly and have
the instrument warmed up for at least 30 minutes, please per-
form the following steps:
- Perform a Factory Preset to start with a dedicated setup:
- Press [RECALL] [9]
- Prepare the instrument to calibrate on all frequencies:
- Press [SELECT] [3]
- Change to (Mode) using the rotary knob